A filter apparatus for filtering suspended solids is disclosed in U.S. Pat. Nos. 5,374,360, issued to Weis, and 5,362,401, issued to Whetsel. The filter apparatus shown there comprises a tank having an inlet and an outlet, one or more filter frames positioned between the inlet and outlet, and filter material supported within the filter frames. During a filtering operation, liquid influent is passed through the filter material such that the filter material filters out suspended solids from the influent stream. The filtered solids are deposited on the filter material while the filtered liquid stream is discharged from the tank as effluent.
To clean the filter material of filtered solids without having to remove the filter frames from the tank, it is known to provide the filter apparatus with a suction generating device having one or more suction heads positioned adjacent a surface of the filter material. It is also known to provide spray nozzles for directing a high-pressure liquid spray to another surface of the filter material. Typically, the spray nozzles are fed by a pump which draws water from a source external to the filter apparatus. The known method of cleaning the filter material is a two-stage process involving the operation of these two components.
In a first stage called the backwash operation, the suction generating device is operated to draw filtered liquid through the filter material in a reverse or backwash direction. The backwash stream that results acts to dislodge solids which have been deposited on the surfaces of the filter material.
In practice, filtered solids continue to accumulate within the filter material despite successive backwash operations. In a second stage of the cleaning process, the spray nozzles are operated to direct a high-pressure liquid spray to the surface of the filter material. The spray acts to dislodge solids which are deeply entrained in the filter material and cannot be removed by the backwash operation. Typically, the dislodged solids either sink to the tank bottom or dissipate into the liquid influent to be re-filtered and eventually removed from the filter material by the backwash operation.
Nevertheless, the high pressure spray does not completely clean the filter material of all filtered solids, biogrowth, oily buildup, and other foreign matter. In time, so much of these contaminants accumulate on the filter material that the filter apparatus has to be taken out of service for intensive washing of the filter material.